1. Field of the Invention
The present invention relates to a stacked type piezoelectric element usable as a source of drive force for an injector.
2. Description of the Related Art
An injector (fuel injection device) of an internal combustion engine of an automobile etc. is designed to inject fuel by operating a valve element of a three-way valve or two-way valve connected to a common rail storing high pressure fuel to switch the opening state of the fuel passages and change the state of pressure imparted to the nozzle needle to open the needle nozzle.
A solenoid valve etc. is generally used as the source of drive force for operating the valve element. Recently, however, attempts have been made to use a stacked type piezoelectric element as the source of drive force, as disclosed in for example Japanese Unexamined Patent Publication (Kokai) No. 11-229993, for the purpose of finer control of the source of drive force and precision control of the state of fuel injection.
While injectors using piezoelectric elements as sources of drive force have been proposed as in Japanese unexamined Patent Publication (Kokai) No. 11-229993, none have yet been commercialized
Depending on the type of injector, the injection of a high pressure fuel of for example over 100 MPa is required. Therefore, reliability under a tough environment of use is required in a piezoelectric element for an injector.
When using a conventional piezoelectric element utilized in other fields for an injector of an internal combustion engine of an automobile etc., the piezoelectric element has often not been able to withstand practical use such as, for example, cracking of the piezoelectric element, disconnection in the internal electrode layers, short-circuits due to cracking, etc. in a short time under such a tough environment of use.
Further, for such high pressure fuel injection, a piezoelectric element having an extremely high performance of a displacement of at least 20 xcexcm and a generated force of at least 1000N is necessary. From this viewpoint as well, it has been difficult to use conventional piezoelectric elements for such injectors.
An object of the present invention is to provide a piezoelectric element which can optimally be used as a source of drive force for an injector.
According to a first aspect of the present invention, there is provided a piezoelectric element built into an injector for generating drive force for the injector, comprising a plurality of alternately stacked piezoelectric layers expanding and contracting in accordance with an applied voltage and internal electrode layers for supplying the applied voltage, each piezoelectric layer containing voids; a total thickness, in a stacking direction, of voids contained in one piezoelectric layer being not more than ⅓ of a thickness of the piezoelectric layer in the stacking direction; and a thickness of any single void in the stacking direction being not more than 50 xcexcm.
Preferably, the piezoelectric element can drive the injector at least 2 xc3x97109 times.
According to a second aspect of the present invention, there is provided a piezoelectric element built into an injector for generating drive force for the injector, comprising a plurality of alternately stacked piezoelectric layers expanding and contracting in accordance with an applied voltage and internal electrode layers for supplying the applied voltage; a thickness of the internal electrode layer being not more than 0.11 times a thickness of the piezoelectric layer.
Preferably, the thickness of the internal electrode layer is not more than 12 xcexcm.
According to a third aspect of the present invention, there is provided a piezoelectric element built into an injector for generating drive force for the injector, comprising a plurality of alternately stacked piezoelectric layers expanding and contracting in accordance with an applied voltage and internal electrode layers for supplying the applied voltage; a relation of 70%xe2x89xa6S2/S1xe2x89xa698% standing between an area S1 of a stacking surface of the piezoelectric layer perpendicular to a stacking direction of piezoelectric layers and an area S2 of an internal electrode layer covering the stacking surface.
Preferably, the piezoelectric element is divided in the stacking direction of the piezoelectric layers into a drive part, buffer parts arranged so as to sandwich the drive part, and dummy parts arranged so as to sandwich the buffer parts; the drive part, buffer parts, and dummy parts being configured so that the amounts of expansion and contraction at the time of application of voltage becomes smaller in the order of the drive part, buffer parts, and dummy parts; and a thickness of a piezoelectric layer at the buffer parts being 1.1 to 5.0 times a thickness of a thinnest piezoelectric layer in the drive part.
Preferably, the thickness of a piezoelectric layer in the drive part is not more than 280 xcexcm.
Preferably, a buffer part includes a plurality of piezoelectric layers.
Preferably, a plurality of drive parts and buffer parts are alternately arranged.
Preferably, a thickness of a piezoelectric layer in a dummy part is not less than 3 times a thickness of a thinnest piezoelectric layer in the drive part.
Preferably, the internal electrode layer is comprised of a material containing Ag and Pd.
Preferably, the internal electrode layer is comprised of a material containing Ag and Pd; and the Agxe2x80x94Pd containing material contains at least 10 wt % of Pd with respect to the total weight of Ag and Pd.
Preferably, sintered particles comprising the Agxe2x80x94Pd containing material include at least 80% of particles of a particle size of not more than 10 xcexcm.
Preferably, each piezoelectric layer is mainly comprised of lead zirconium titanate.
Preferably, each piezoelectric layer contains Mn, the content of Mn being 0.005 to 0.4 wt % with respect to the piezoelectric layer.
Preferably, the lead zirconium titanate basically comprises a Pb(Y0.5Nb0.5)O3xe2x80x94PbTiO3xe2x80x94PbZrO3xe2x80x94based solid solution; ratios of ingredients in the three-ingredient solid solution are 0.5 mol % less than Pb (Y0.5Nb0.5) O3 xe2x89xa63 mol %, 42 mol % less than PbTiO3  less than 50 mol %, and 47 mol % less than PbZrO3 xe2x89xa657.5 mol %; substituent groups of Pb by Sr are present in more than 5 mol % and not more than 15 mol %; a content of Nb2O5 to lead zirconium titanate is not more than 1 wt %; and a content of Mn2O3 to lead zirconium titanate is not less than 0.01 wt % and less than 0.5 wt %.
Preferably, sintered particles comprising the piezoelectric layer have a particle size of not more than 8 xcexcm.
Preferably, the piezoelectric element drives the injector a plurality of times for each combustion stroke of an internal combustion engine.